Dewaxing is often required when paraffinic oils are to be used in products which need to have good fluid properties at low temperatures e.g. lubricating oils, heating oils, jet fuels. The higher molecular weight straight chain normal and slightly branched paraffins which are present in oils of this kind are waxes which are the cause of high pour points in the oils. If adequately low pour points are to be obtained, these waxes must be wholly or partly removed. In the past, various solvent removal techniques were used e.g. propane dewaxing, MEK dewaxing; but, the decrease in demand for petroleum waxes, together with the increased demand for gasoline and distillate fuels, has made it desirable to find economic processes which convert waxy components into other materials of higher value. Catalytic dewaxing processes can achieve this by selectively cracking the longer chain paraffins, to produce lower molecular weight products which may be removed by distillation. Processes of this kind are described, for example, in The Oil and Gas Journal, Jan. 6, 1975; pages 69 to 73 an U.S. Pat. No. 3,668,113.
It is also known to produce a high quality lube base stock oil by subjecting a waxy crude oil fraction to solvent refining, followed by catalytic hydrodewaxing (HDW) over ZSM-5, with subsequent hydrotreating (HDT) of the lube base stock, as taught in U.S. Pat. No. 4,181,598, the entire contents of which is incorporated herein by reference.
In order to obtain the desired selectivity, the catalyst has usually been a zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins, but which generally excludes more highly branched materials, cycloaliphatics and heavy aromatics. Shape-selective zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes and their use is described in U.S. Pat. Nos. 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282, 4,247,388, 4,257,872, 4,313,817, 4,436,614, and 4,490,242, incorporated herein by reference. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. A hydrocracking process employing zeolite beta as the acidic component is described in U.S. Pat. No. 3,923,641.
Dewaxing processes of this kind function by cracking waxy components to form lower molecular weight materials, including olefins and other unsaturated compounds which contribute to deactivation of the catalyst. Cracking products, especially lower olefins, tend to further degrade to form carbonaceous deposits on the catalyst. Coking deactivates the catalyst requiring the process temperature to be raised in order to achieve the desired degree of conversion. As the aging of the catalyst has resulted in the process temperature increasing to an upper limit, the production process is interrupted to permit periodic oxidative regeneration of the catalyst. Frequent shutdown of the production unit for catalyst regeneration can render the dewaxing process less economic.
Prior work has established the value of metal-exchanged and/or impregnated zeolites, especially acidic Ni-ZSM-5 zeolite, as a hydrodewaxing catalyst. Pd-exchanged ZSM-5 has a lower aging rate than other Group VIII metals, but this requires extra catalyst processing beyond that of the economic standard zeolites employed in commercial HDW processes. It has also been proposed to admix a hydrogenation catalyst, such as palladium on alumina, with a standard HDW catalyst; however, this poses problems in catalyst loading and regeneration techniques. Density differences between the two catalysts make mixed loading difficult.
It is an object of the present invention to improve the catalytic hydrodewaxing process by extending the useful production cycle. This can be achieved by the discovery that staged conversion in a multi-bed dewaxing reactor system with an intermediate catalytic hydrotreating zone operatively connected between alternating beds of dewaxing catalyst can improve performance, resulting in improved aging characteristics.